Background gas enters the mud circulating system as the formation is drilled by the drilling bit and usually maintains a steady but low level. Additionally, as shales are circulated up the hole, the reduction in pressure explodes the shale particles releasing gas into the drilling mud. These are the most common sources of gas.
If a low differential pressure exists from a combination of low-density mud and Abnormal pressure, gas will enter the borehole and increase the amount of background gas in the mud.
Pixler (1945) recommended the use of gas measurements for the detection of overpressures and for warnings of impending blowouts. Goldsmith (1972) stated that most impermeable shales would contain some gas, while abnormally pressured shales often contain large quantities of gas. Fertl (1973) explains this by stating that comparatively free gas diffusion is possible through clay, as a function of the median pore size of clays or silty clays, and the varying diameter of gas molecules.
Factors Affecting Measurement Of Background Gas
Since overpressured shales have high porosity, diffusion will be enhanced, resulting in shale gas being found over long impermeable shale sections. Low salinity and high pressures increase the amount of solution gas in formation waters. Background gas will normally increase in a transition zone as the porosity increases, hence a higher gas content. Additionally, the increased ROP will release more cuttings, freeing more gas, and the reduction in the overbalance will cause levels to increase. This latter point may be important in identifying transition zones when the overbalance is small. Gas readings may be masked, and analysis is impossible where this is too high.
Background Gas Levels
Background gas levels should be continuously monitored and plotted. The operator must be aware of trends, changes to trends, and the controlling factors associated with background gas levels.
Many factors will affect the background gas levels, such as:
- Mud Properties
- Pump Rate
- Gas Composition
- Mud Weight
Evaluation of Gas Pressure
The correlation of the background gas with changes in the mud weight can give an accurate indication of the differential pressure and, consequently, the formation pressure. For example, suppose a small mud weight increase suddenly decreases high background gas levels with associated connection gas peaks. In that case, it is reasonable to assume that the formation pressure is slightly below the new Equivalent Circulating Density ECD.